Sensing circuit



June 9, 1964 STAB/USED D. W. AVIS SENSING CIRCUIT Original Filed June30, 1960 POWER SUPPL Y Pom/5m .V. INPUT men/mus Inventor 0. w A visDivided and this application Oct. 23, 1961, Ser. No.

146,908 Claims priority, application Great Britain July 6, 1959 4Claims. (Cl. 317--148.5)

The present invention relates to a sensing circuit for producing anindication in response to a small D.C. current or voltage. Thisapplication is a division of application Serial No. 39,919 filed'June30, 1960, and now abandoned.

According to the present invention a sensing circuit comprises atransistor amplifier to which a small DC. current or voltage to beindicated is applied and comprising a plurality of stages and a relay orthe like connected in the output circuit of the final transistoramplifying stage which relay or the like is operated upon the inputcurrent or voltage exceeding a certain small value, said circuit beingpolarised so that the relay or the like is only operated when the inputcurrent or voltage is of a particular polarity and said relay or thelike operating an indicating device.

The transistor amplifier may comprise a common emitter input stagefeeding a common collector intermediate stage which in turn feeds acommon emitter output stage. The transistor amplifier may include atemperature stablising circuit consisting of a further transistor forneutralising fluctuations in the collector current of the inputtransistor with changes in temperature. The sensing circuit possesses ahigh insulation to earth and very low leakage in order to make itresponsive to very small input voltages or currents.

In order that the invention may be more fully understood reference willnow be made to the accompanying drawing which is a circuit diagram ofone embodiment of a sensing circuit according to this invention.

Referring to the drawing, the embodiment of sensing circuit illustratedhas both input terminals completely isolated from earth and a polarisedinput sensitivity of the order of volt at about 1 a. so that it may bewired into a circuit rather in the same way as a moving coil metermovement and produce an output indication, by operation of a relay, whenan input current or voltage of a certain polarity applied theretoexceeds a very small value. The high sensitivity on both voltage andcurrent makes the circuit extremely versatile as it may be connectedinto a bridge circuit for voltage measurements and a bucking circuit forcurrent measurements. These methods increase the stability by threeorders of magnitude when voltages of about 100 v. and currents of about1 ma. are being dealt with. Since the sensing circuits must have a highresistance to earth, they are built on panels of low leakage insulatingmaterial and each panel is provided with its own individual stabilisedpower pack built on the panel. The power transformer and the relayemployed. must themselves possess extremely low leakage between windingsand their cores. To

this end the power transformer employed is constructed with a singlelayer of heavily insulated wire for a primary working at less than 0.5volt R.M.S. input. All the individual power transformers are fed in turnfrom a single very low voltage mains transformer.

The input transistor TRl is an RF. type of component which will operateat low collector voltage thereby ensuring a low value of collectorleakage current (Ico), a desirable feature in this application. Thisstage operates in the common emitter configuration, the input beingbetween base and emitter. Transistor TR2 is the United States Pat Ouseful improvement is to be obtained.

3,136,928 Patented June 9, 1964 ICC second stage and is arranged in acommon collector circuit feeding transistor TR3 arranged as the commonemitter output stage with the relay RL connected in the collectorcircuit. The collector current of TR3 is arranged to be about 15 ma. atfull drive and is ample to operate the 350 ohm relay RL. The relaycontacts close at about 10 ma. which corresponds to an input current ofabout 0.3 a. to the base electrode of TRl. The Ico current of TRlfluctuates quite markedly with temperature and must be satisfactorilyneutralised if the unit is to be temperature stable. If a resistor whosevalue is independent of temperature is used, then neutralising can onlybe correct at one temperature. If feedback is introduced the overallgain must be impaired if a It is possible to use a temperature dependentresistor, but these are only obtainable in a few types and cannot havetheir characteristics changed so as to obtain the optimum values. In thecircuit as shown, a further transistor TR4 is used for temperaturestabilisation, whose base is fed from a potential divider P1 connectedbetween its collector and emitter. By selecting appropriate values forthe portions of potentiometer P1 it is possible to obtain a wide rangeof thermal drift compensation. If desired the potentiometer mayconstitute a single resistor having an adjustable tapping which is setto the desired value.

The input circuit is effectively polarised by a diode D1 which has a lowforward resistance compared to the input resistors R1, R2 and R3.Reverse currents flow through diode D1 and not into the base of TRI.Condensers C1 and C2 in conjunction with the input resistors areprovided so as to make the circuit insensitive to transient surges.Furthermore they prevent damage to the diode or transistors from theseor any other transients.

When properly adjusted the circuit is insensitive to input current untilits value is about half that required to close the relay RL, whichcomponent has adjustable stops fitted to the armature, the result beinga very small operating differential.

in order to obtain the high insulation to earth and low leakageperformance of the circuit, the insulation of the relay winding and thepower supply input have to be very effective.

I claim:

1. A sensing circuit for producing an indication in response to a smallDC. current or voltage and consisting of a transistor amplifier to whichthe small current or voltage to be indicated is applied, said amplifiercomprising a common emitter input stage, a common collector intermediatestage and a common emitter output stage, a diode device connected acrossthe input to said common emitter input stage to polarise said circuit,relay means connected to said common emitter output stage andoperatedupon the input current or voltage exceeding a predetermined small value,and temperature stabilising means comprising a further transistorconnected in the emitter circuit of said common emitter input stage forneutralising fluctuations in the collector current of said inputtransistor with changes in temperature.

2. A sensing circuit for producing an indication in response to a smallDC. current or voltage and consisting of a transistor amplifier to whichthe small current or voltage to be indicated is applied, said amplifiercomprising first, second and third transistors each having an emitter, abase and collector, a stabilised power supply for said transistors, saidfirst transistor being connected as a common emitter input stage andhaving input terminals connected to its base and emitter, a diode deviceconnected across said input terminals to polarise said circuit so thatit only responds to inputs of a chosen polarity, a fourth transistorconnected in the emitter circuit of said first transistor and formingtemperature stabilising means for neutralising fluctuations in thecollector current of said first transistor with changes in temperature,a connection from the emitter of said first transistor to the base ofsaid second transistor which is connected as a common collectorintermediate stage, connections from the collector of said first andsecond transistors to said stabilised power supply, a connection fromthe emitter of said second transistor to the base of said thirdtransistor connected as a common emitter output stage, and relay meansconnected between the collector of said third transistor and saidstabilised power supply, said relay means being operated upon the inputcurrent or voltage applied to said circuit exceeding a predeterminedsmall value.

3. A circuit as claimed in claim 2, in which the collector of the fourthtransistor is connected to the emitter of the first transistor and thebase of said fourth transistor is connected to an intermediate point ona potential divider connected between its emitter and collector.

4 4. A circuit as claimed in claim 3, in which the input circuit to thefirst transistor comprises a resistor connected between each inputterminal and said diode device, a first condensed connected across saidinput terminals, and a second condenser connected between one terminalof said first condenser and the emitter of said fourth transistor.

References Cited in the file of this patent UNITED STATES PATENTS2,864,904 Jensen Dec. 16, 1958 2,877,310 Donald Mar. 10, 1959 3,008,091Van Overbeek et al. Nov. 7, 1961 3,075,151 Murray Jan. 22, 1963 FOREIGNPATENTS 564,571 Canada Oct. 14, 1958 OTHER REFERENCES Bohr, SensitiveRelay Circuits, Radio-Electronics, pp. 112, 114, 115, January 1958. r

1. A SENSING CIRCUIT FOR PRODUCING AN INDICATION IN RESPONSE TO A SMALLD.C. CURRENT OR VOLTAGE AND CONSISTING OF A TRANSISTOR AMPLIFIER TOWHICH THE SMALL CURRENT OR VOLTAGE TO BE INDICATED IS APPLIED, SAIDAMPLIFIER COMPRISING A COMMON EMITTER INPUT STAGE, A COMMON COLLECTORINTERMEDIATE STAGE AND A COMMON EMITTER OUTPUT STAGE, A DIODE DEVICECONNECTED ACROSS THE INPUT TO SAID COMMON EMITTER INPUT STAGE TOPOLARISE SAID CIRCUIT, RELAY MEANS CONNECTED TO SAID COMMON EMITTEROUTPUT STAGE AND OPERATED UPON THE INPUT CURRENT OR VOLTAGE EXCEEDING APREDETERMINED SMALL VALUE, AND TEMPERATURE STABILISING MEANS COMPRISINGA FURTHER TRANSISTOR CONNECTED IN THE EMITTER CIRCUIT OF SAID COMMONEMITTER INPUT STAGE FOR NEUTRALISING FLUCTUATIONS IN THE COLLECTORCURRENT OF SAID INPUT TRANSISTOR WITH CHANGES IN TEMPERATURE.